The receivers intended for the detection of radiofrequency signals, for example of radar type, must be capable of monitoring wide frequency bands notably in the microwave range. They must be able to detect, for example, radar signal pulses. These pulses may exhibit “chirp” or phase code type modulations, these modulations being used by the radar to compress the pulse on reception. The pulses may also be unmodulated, except by the all or nothing modulation defining the pulse. Among other things, the function of these receivers is to characterize the intercepted pulses by estimating parameters such as the time of arrival, the pulse width, the center frequency, the presence of modulation within the pulse and, where appropriate, the modulation type.
Although digital processing operations are usually included in the architecture of these receivers, the current solutions for estimating the frequencies of the received signals, hereinafter in the description designated by the acronym IFM (Instantaneous Frequency Measurement), are mostly based on analogue techniques.
As an example, one existing solution allowing for an instantaneous measurement of the frequency of the received signal is based on the creation of a regime of standing waves in a propagation line attacked at one of its ends by the signal and at the other by the delayed signal. The periodicity of the nodes and of the antinodes gives a rough measurement of the frequency of the received signal. The measurement of the position of the nodes and of the antinodes distributed along this line gives a fine measurement of the frequency of the received signal. This type of IFM is called frequency meter with spatial sampling.
Another solution of the state of the art, which is very widely used, is based on self-correlators or phase meters. The principle in this case is to directly measure the phase difference φ induced by a delay line and deduce the frequency therefrom. A number of stages, placed in parallel, are generally needed to provide the desired frequency band and accuracy.
The receivers implementing these solutions use analogue functions and are therefore subject to drifts such as delay variations as a function of temperature, level or phase shift measurement imperfections. This leads to a bulky and very costly architecture.
More recently, a French patent application relating to a frequency measurement wideband digital receiver filed under the number 06/01205 describes, unlike the previous two examples, a way of digitizing the signal on input, and of performing all the processing operations digitally. The digitization is performed at a sampling frequency well below the Shannon criterion. This is reflected in an ambiguous frequency measurement Fmeasurement, also called fine frequency hereinafter in the description, said frequency being able to be described by the following expression:Fmeasurement=±(Freal−j×Fe)+δF  (1)in which Freal is the real frequency of the received signal, j a positive integer, Fe the sampling frequency of the system and δF is the measurement error due mainly to the signal-to-noise ratio.
To resolve this ambiguity, N measurement channels are used in parallel, with offset sampling frequencies. The N ambiguous frequency measurements are associated to resolve said ambiguity and obtain a measurement of the real frequency of the incident signal. This measurement is called final frequency hereinafter in the description.
The detection of phase jumps is currently absent from most of the wideband IFM receivers used for the detection of signals such as radar pulses. This means that it is difficult to fully characterize a received signal pulse and phase codes, for example, cannot be detected.